Relationship between structure of the 5' noncoding region of viral mRNA and efficiency in the initiation step of protein synthesis in a eukaryotic system.

Abstract

To determine whether the rate of protein synthesis is controlled by the structure ofmRNA near its 5' terminus, protein-synthesizing ability, especially in its initial stage, was compared among RNAs of plant viruses. Those viruses used here contain several definite pieces of single-stranded RNA. Each of these RNAs acts as a messenger. Cucumber mosaic virus (CMV) RNA 5 synthesizes a small amount of a protein, Mr 7000, in an in vitro protein-synthesizing system from wheat germ or reticulocyte. Brome mosaic virus (BMV) RNA 4 synthesizes a large amount of a coat protein under the same conditions. Both RNAs have the same 5'-cap structure and a short noncoding region (10 nucleotides in CMV RNA 5 and 9 in BMV RNA 4) between the 5' terminus and the initiation codon AUG. A sequence complementary to the 3' terminal of 18S ribosomal RNA is contained in BMV RNA 4 but is not apparent in CMV RNA 5. Formation of the initiation complex for protein synthesis by the 5'-terminal-labeled mRNA of cytoplasmic polyhedrosis virus was inhibited by the addition of unlabeled BMV RNA 4 whereas it was only slightly inhibited by unlabeled CMV RNA 5. BMV RNA 4, which has a sequence complementary to rRNA, can form the initiation complex more easily than CMV RNA 5. It is concluded that an apparent complementary sequence to the 3' terminal of 18S rRNA in the 5' noncoding region of eukaryotic mRNA and the 5'-cap structure enhance the rate of initiation complex formation in protein synthesis.Usually, prokaryotic mRNA is polycistronic whereas eukaryotic mRNA is monocistronic. The latter carries the cap structure at the 5' terminal and protein synthesis begins from the initiation codon AUG (1). In prokaryotic mRNA, a sequence complementary to the 3'-terminal part of 16S rRNA is located in front of the initiation codon of protein synthesis, and it is considered to bind to ribosomes to form an initiation complex as the first step in protein synthesis (2-5).The nucleotide sequences of the 3'-terminal region of 18S eukaryotic rRNA have been determined for mouse, rat, rabbit, hen, silkworm, wheat, and barley (6-8). These are almost identical to one another and similar to prokaryotic 16S rRNA sequences. Thus, this part of eukaryotic rRNA should have a secondary structure similar to the prokaryotic one, taking a hairpin structure containing the m2A-m2A sequence in the loop part (6-8). According to the psolaren crosslinking experiment of Nakashima et al. (9), the 5'-terminal part of eukaryotic mRNA interacts with the 3'-terminal part of 18S rRNA as seen in prokaryotes. But the eukaryotic sequence of 18S rRNA near the 3' terminus lacks the sequence C-C-U-C-C, which is complementary to mRNA in prokaryotes. Therefore, the mechanism of initiation interaction of mRNA with ribosomes should be different in prokaryotes and in eukaryotes. The 5'-terminal noncoding regions ofthe different eukaryotic mRNAs vary not only in length and secondary structure but also in nucleotide sequence. Some contain a nucleotide sequence complementary to the 3'-terminal part of...

Comparison of initiation of protein synthesis in procaryotes, eucaryotes, and organelles.

“…Before concluding that the putative Shine-Dalgarno sequence plays no role, however, it will be necessary to quantitate the yield of protein from the wild-type and mutant viral mRNAs. Yamaguchi et al (537) recently attempted to correlate the translational efficiency of two plant virus mRNAs with the presence or absence of a sequence complementary to the 3' end of 18S rRNA, but they presented no evidence that the AUG triplet on which they focused is the functional initiator codon in cucumber mosaic virus RNA-5. Thus, the case for a Shine-Dalgarno interaction in eucaryotes is not strong.…”

Complete Nucleotide Sequences of Two Adjacent Early Vaccinia Virus Genes Located Within the Inverted Terminal Repetition

“…This complementarity is thought to facilitate the binding of mRNA to ribosomes. The situation in eucaryotes is less certain since the presence of a sequence complementary to 18S rRNA is variable (17,46). For this reason, it was of interest to examine the leader sequences of early vaccinia virus mRNAs.…”

Distribution and partial properties of three viruses infecting cucumber in West Iran and their reservoir weed hosts

“…The CMV-Ar isolate from Lorestan province fell into subgroup IA, whereas CMV-Ar and CMV-Rs isolates fell into subgroup IB (Figure 1). The conserved hexanucleotide sequence (5′-TCGTAG-3′) upstream of the CP gene start codon that is involved in 18S ribosomal RNA binding in subgroup I (Yamaguchi et al 1982) was only found in subgroup IA isolates, but not in any of the subgroup IB isolates (data not shown).…”